This invention relates generally to platforms from which offshore operations, such as petroleum drilling and production, can be carried out and methods for installing or deploying these offshore platforms. The invention is particularly concerned with (1) methods for deploying, normally in relatively deepwater, floating platforms comprising two or more separately fabricated modules or structures and (2) the resulting platforms whose low heave, pitch and roll motions enable them to support surface wellhead equipment.
As hydrocarbon reserves decline, the search for oil and gas has moved offshore into increasingly deeper waters where economic considerations and physical limitations frequently militate against the use of platforms supported on the ocean or sea floor. Thus, most offshore drilling and production in deep water is conducted from floating platforms that support the drill rig, derrick, and associated drilling and production equipment. The three types of floating platforms that see the most use in deepwater are semisubmersible platforms, tension leg platforms (TLPs), and spars.
Semisubmersible floating platforms typically consist of a flotation hull usually comprising four or more large diameter vertical columns supported on two or more horizontal pontoons. The columns extend upward from the pontoons and support a platform deck. The flotation hull, when deballasted, allows the platform to be floated to the drill site where the hull is ballasted with seawater to submerge it such that the deck remains above the water surface. The platform is held in position by moorings lines anchored to the sea floor. Partially submerging the hull beneath the water surface reduces the effect of environmental forces, such as wind and waves, and results in a relatively stable work deck. Although the semisubmersible platform is stable for most drilling operations, it still exhibits a relatively large heave response to the environment that makes the use of surface wellheads (wellheads located above the water surface) undesirable because of the complexity and cost of riser tensioners and other clearance systems required to permit relative movement between the riser pipes and platform. Instead, the wellheads are typically located on the seafloor, and relatively complex and costly subsea equipment is used to produce hydrocarbons. However, the cost of accessing the wellheads for servicing and workovers becomes more difficult and costly as the water depth increases, thereby making the use of conventional semisubmersibles in deep water somewhat undesirable.
Tension leg platforms (TLPs) are also used to produce hydrocarbons in deep water. These platforms are moored to the ocean floor using semirigid or axially stiff (not axially flexible), substantially vertical tethers or tendons (usually a series of interconnected tubulars). The TLP platform is comprised of a deck and hull similar in configuration and construction to the semisubmersible platform. The hull provides excess buoyancy to support the deck and to tension the tethers and production risers. The deck supports drilling and production operations. The use of axially stiff tethers tensioned by the excess buoyancy of the hull to moor the platform tends to substantially eliminate heave, roll and pitch motions, thereby permitting the use of surface wellheads and all the benefits that accompany their use. However, heave restraining the entire platform, including the drilling rig, crew quarters and equipment, requires a substantial amount of additional buoyancy and tether steel, which in turn substantially increases the cost of the TLP.
Another type of floating structure used in offshore drilling and production operations is a spar. This type of structure is typically an elongated, vertically disposed, cylindrical hull that is buoyant at the top and ballasted at its base. The hull is anchored to the sea floor by flexible taut or catenary mooring lines. Although the upper portion of a spar""s hull is buoyant, it is normally not ballastable. Substantially all the ballast is located in the lower portion of the hull and causes the spar to have a very deep draft, which tends to reduce heave, pitch and roll motions. The main problem with the use of spar platforms is the difficulty in deploying them in deep water. The elongated hull must be towed to the desired offshore location on its side and then upended in the water so it can be vertically oriented. After it is upended, its deck and associated equipment must be placed on the top of the hull. Both of these operations require the use of a large floating crane and other equipment at the offshore location, thus making the installation a complex and expensive endeavor.
It is clear from the above discussion that the three types of platforms commonly used in deepwater exploration and production have significant disadvantages. Thus, there exists a need for other platform designs that result in structures that not only possess low heave, pitch and roll motions but are also relatively inexpensive and simple to build and easy to deploy in relatively deep offshore waters.
In accordance with the invention, it has now been found that a floating platform or other apparatus can be more easily constructed and deployed in a body of water if it is comprised of two or more buoyant and ballastable structures or hulls that are separately fabricated in conventional shipyards or other fabrication facilities and then individually floated to the desired location in the body of water. Here, the lower structure is anchored to the floor of the body of water, usually using flexible and substantially non-vertical mooring lines, and then ballasted down until completely submerged. After the lower buoyant and ballastable structure has been anchored and completely submerged in the body of water, an upper structure is floated over the lower structure, and both structures are selectively ballasted and/or deballasted until the top of the lower structure mates with the bottom of the upper structure under the water surface to form a floating apparatus that can support a deck or platform from which offshore operations are conducted. The lower structure remains completely submerged in the water without touching the bottom of the body of water.
The resulting floating apparatus comprises an uppermost buoyant and ballastable structure partially submerged in the water without contacting the floor of the body of water and a lower buoyant and ballastable structure, which typically has a height greater than about 50% of the height of the uppermost structure, that is completely submerged in the water without contacting the floor of the body of water. The bottom of the uppermost structure is fixedly mated to the top of the lower structure, and the lower structure is anchored to the floor of the body of water, usually with flexible and non-vertical mooring lines. The uppermost structure typically supports a deck from which drilling, production, and workover operations are carried out. The relatively deep draft, usually greater than about 150 feet, of the combined structures coupled with the moorings used on the lower structure results in substantially reduced heave, pitch and roll motions and thereby makes it feasible to employ surface wellheads. Normally, it is not necessary for obtaining the desired heave, pitch and roll responses of the combined structures to anchor the uppermost structure to the floor of the body of water with moorings of any kind. However, the uppermost structure may contain winches or other devices for tensioning the mooring lines used to anchor the lower structure.
In some instances, especially when it is desired to provide oil and/or gas storage capabilities to the invention, more than two separate buoyant and ballastable structures can be utilized in the floating apparatus. Such a system can be constructed by fabricating the additional structure or structures in the shipyard and floating them to the desired offshore location where they are selectively ballasted and/or deballasted as described above such that the top of one structure mates with the bottom of another. The resulting apparatus will then comprise two or more structures completely submerged in the water with the uppermost structure only partially submerged and supporting a deck from which offshore operations can be conducted. Typically, only the lowermost structure will be anchored to the floor of the body of water, usually with flexible, non-vertical mooring lines. In this apparatus, the lowermost structure(s) can be designed to store oil and/or gas, and all of the structures combined may have a draft of as much as about 150 to 400 feet.
The apparatus and method of the invention have significant advantages over conventional offshore platforms and installation methods. The individual modules or structures comprising the apparatus of the invention can be made in simple shapes (e.g., square and rectangle boxes) and in relatively small sizes (e.g., heights usually less than about 150 feet) that allow the structures to be fabricated in conventional shipyards with conventional equipment. The uppermost structure can be built in the shipyard with the deck and associated drilling, production, and/or workover equipment preinstalled so that vertical lifting devices are not needed offshore to fit the platform and its equipment to the supporting structure. Furthermore, since the individual modules or structures are buoyant and ballastable, they can be towed to the desired offshore site without using barges and can be fixed together without the need for heavy lift equipment. Finally, the heave, pitch and roll resistance of the combined modules or structures allows the use of surface completions and wellheads.